R Coronae Borealis star

An R Coronae Borealis star is a luminous, eruptive variable supergiant, rich in carbon and poor in hydrogen, of spectral type F or G, unusual in that it goes into outburst
not by brightening, but by fading. Each minimum is thought to be caused
by carbon-rich material puffed off the star during a pulsation cycle. As
the cloud moves out, it cools and eventually condenses into carbon dust particles, which absorb much of the light coming from the star's photosphere.
Only when the dust has been blown away by radiation
pressure, does the star return to its normal brightness. While at maximum
light, R Coronae Borealis stars are observed to undergo small-scale, Cepheid-like
variations with fluctuations of several tenths of a magnitude and with periods
of 20 to 100 days.

Typical is the behavior of R Coronae Borealis (R CrB), the prototype of
the class. R CrB is an F8 or G0 Ib supergiant whose variability was discovered
nearly 200 years ago by Edward Pigott. Most
of the time, R CrB is at its maximum of around magnitude 6 (see light curve
above). But at irregular intervals it goes, over a period of several weeks,
into a decline of up to 8 magnitudes. The star may remain faint for many
months or have several recoveries and declines in succession. Often the
final rise back to maximum light is slow, taking several months to a year.

The RCB phase is probably fairly short-lived, maybe on the order of 1,000
years, as evidenced by the fact that less than 50 of these stars are known.
Their evolutionary status is uncertain, though there are two main theories:
(1) the Double Degenerate (DD) model or (2) the Final
Helium Shell Flash (FF) model. Both involve the expansion of white
dwarfs to the supergiant phase. The DD model invokes the merging of
two white dwarf stars, while the FF model assumes that a single white dwarf
star expands to the supergiant stage by means of a final
helium flash.